1. Field of the Invention
This invention relates to a process for making circuit boards and the circuit boards made in accordance therewith, and more particularly to a process for making single-sided or double-sided electrical circuit board with coverfilm or covercoat on either the top side, the bottom side, or both.
2. Description of the Prior Art
Single- and double-sided electrical circuit boards with coverfilm are known in the art. The board material is commercially supplied in a roll with a base film prelaminated to a copper layer without access windows in the film with which to allow contact to the copper from the film side. Circuits are made by screening a positive image onto the polyclad substrate; many separate circuits may be screened on a roll of material. The areas of copper left exposed after screening are etched or plated to reveal a circuit design. Next, access windows and a circuit design are punched out of a base film with adhesive on one side, which is aligned (either manually or with the use of guide pins) over the etched circuit. The coverfilm is then heated to tack the coverfilm to the etched circuit, after which it is fully laminated by running the tacked structure through a nip roll laminator or hydraulic press. Finally, the final circuits are punched out from the reel of material. This process is very labor intensive and is not well-suited to automation.
In addition, the manufacture of double-sided circuit boards of this type allowing electrical connection between traces on one side of an electrical circuit board or single-sided circuit boards having access to the conductive traces from both sides has not been economical. Typically, access windows are opened up in the board substrate by sciving or milling the circuit board containing the finished circuit. This method is, however, a very expensive and time-consuming process requiring precision equipment, and does not lend itself to high-volume production. Alternately, plated-through hole (PTH) technology is used. PTH technology requires that access holes be punched or drilled in double-sided circuit board, and that the holes be deburred. A electroless copper process and optional strike is then done, followed by a screen or photoimage negative pattern. Copper plate is then applied, followed by solder or tin plate, stripping, and etching of the board. A further option of strip solder or tin plate may also be applied, followed by a covercoat or coverfilm application, after which the circuit is punched out. This process is very labor intensive and costly, and does not lend itself to automation without heavy capitalization.
Thus, conventional techniques for making circuit board with coverfilm do not lend themselves to high-volume, automated production, and, without expensive, labor-intensive steps, access to traces on one side of the circuit board is limited only to the side of the circuit board bearing those traces.